New Mexico Mineral Symposium — Abstracts

Cooke’s Peak is a prominent feature of the landscape in southwestern New Mexico. It is predominantly a granodiorite intrusive that has pushed up through the Paleozoic sedimentary rocks. Hydrothermal solutions from the intrusion produced the ore bodies localized in suitable limestone units and the accompanying silicification (jasperoid formation). The district was developed for its lead, zinc, and silver deposits although it is most famous to mineral collectors for the exceptional purple and green octahedral fluorite crystals.

The Summit group of mines, in the Cooke’s Peak district, consists of a number of adits, shafts, pits, and trenches many of which are rich in fluorite. While investigating one of the adits, along with Chris Cowan, on the 19th of February, 2016, one of the authors (RSD) found two pieces of rock approximately 6" x 8" which were coated with lustrous, green microcrystals. A second trip by the authors, accompanied by Philip Simmons, recovered only one additional piece.

The specimens were brought to the New Mexico Bureau of Geology and Mineral Resources for examination and analysis. X-ray diffraction analysis of the green crystals resulted in an x-ray diffractogram that matched the mineral sidwillite (MoO3·2H2O). Previous investigations of the Cooke’s Peak district (Griswold 1961; Jicha 1954; Schilling 1965) specifically do not mention the occurrence of molybdenum minerals. Additional examination of the material led to the identification of molybdite (MoO₃) by x-ray diffraction. Visual identification of ilsemannite (Mo₂⁶+ Mo⁴+)O₈·H₂O? and jordisite (MoS₂) were also inferred. The amorphous nature of these two minerals makes confirmation of these species by XRD impossible. Ilsemannite is characterized by blue color (subject to change once exposed and waxy texture. Jordisite was identified by its shiny black, grainy to earthy texture. A few grains, resembling ferrimolybdite (Fe₂³+(Mo⁶⁺O₄)₃?7-8H₂O) were also observed on some samples.